The present invention relates to a fuel cell system preventing damage to fuel cell at reducing electromotive force, e.g., a case in which a fuel cell at operating is stopped suddenly.
Conventionally, as for an art in this field, there is, e.g., Japanese Publication Patent Laid-Open No. 7-78624.
FIG. 2 is a schematic block diagram showing a conventional fuel cell system described in Japanese Patent Publication Laid-Open No. 7-78624.
The fuel cell system has a motor 1 generating driving torque S1. A compressor 2 is connected to the motor 1. The compressor 2 has a function taking in and compressing cathode active material A (e.g., oxygen, air or the like) and supplying cathode active material S2 whose amount depends on the number of revolutions of the motor 1. The cathode active material S2 is taken in to a fuel cell 3. A fuel blower 4 is provided in the fuel cell system and takes in anode active material F (e.g., fuel of hydrogen gas easy to undergo oxidation) and sends out anode active material S4. The anode active material S4 is taken in to the fuel cell 3. The fuel cell 3 has a cathode side gas chamber 3a, a cathode 3b, an anode side gas chamber 3c, an anode 3d and an electrolyte layer 3e between the cathode 3b and the anode 3d. The fuel cell 3 takes in the cathode active material S2 to the cathode side gas chamber 3a and takes in the anode active material S4 to the anode side gas chamber 3c. Moreover, the fuel cell 3 discharges reaction product S3a, S3c and S3e from the cathode side gas chamber 3a, the anode side gas chamber 3c and the electrolyte layer 3e, respectively, and generates electromotive force S3 between the cathode 3b and the anode 3d. Load L is connected to the cathode 3b and the anode 3d. The reaction product S3a is discharged via a turbine 5. The reaction product S3c is discharged by controlling pressure by a control valve 6.
Next, operation of FIG. 2 will now described.
The cathode active material A is taken in to the compressor 2 and compressed, and the cathode active material S2 whose amount depends on the number of revolutions of the motor 1 is sent out from the compressor 2. The cathode active material S2 is taken in to the fuel cell 3. The anode active material F is taken in to the fuel blower 4 and the anode active material S4 is sent out to the fuel cell 3. The fuel cell 3 takes in the cathode active material S2 the cathode side gas chamber 3a and takes in the anode active material S4 to the anode side gas chamber 3c. Moreover, the fuel cell 3 discharges the reaction product S3a, S3c and S3e from the cathode side gas chamber 3a, the anode side gas chamber 3c and the electrolyte layer 3e, respectively, and generates electromotive force S3 between the cathode 3b and the anode 3d. 
The electromotive force S3 is controlled on the basis of the number of revolutions of the motor 1 and opening of the control valve 6 and supplied to the load L. The reaction product S3a is discharged via the turbine 5 and the reaction product S3c is discharged by controlling pressure by the control valve 6.
However, the conventional fuel cell system in FIG. 2 has a following problem.
FIG. 3 is a characteristic view showing a generation state of overshoot in pressure of the cathode active material S2 and the reaction product S3a in the cathode side gas chamber 3a in FIG. 2. A vertical axis is pressure and a horizontal axis is time.
In the fuel cell system in FIG. 2, when a command for changing the electromotive force S3 is input, the time required for reducing the number of revolutions of the motor 1, from 8000 rpm to 0 rpm of a target value is 1 second and the time required for reducing the opening of the control valve 6, e.g., from 80xc2x0 to 0xc2x0 of a target value is 0.01 second. Specifically, before the motor 1 is stopped the control valve 6 is closed. Therefore, as shown in a characteristic curve C1 in FIG. 3, pressure of the cathode active material S2 and the reaction product S3a in the cathode side gas chamber 3a is P1 kPa at operating. After a lapse of T1 second since an operation stopping command (i.e., 0 second), the pressure is P2 kPa and overshoot is generated. The pressure is reduced gradually and becomes P3 kPa (where P2 greater than  greater than P3). When overshoot is generated, cathode-anode differential pressure between the cathode side gas chamber 3a and the anode side gas chamber 3c is wider than a permissible value and there is a case in which the fuel cell 3 is damaged and destroyed. To solve this problem, Japanese Patent Publication Laid-Open No. 7-78624 proposes a fuel cell system as shown in FIG. 4.
FIG. 4 is a schematic block diagram showing another conventional fuel cell system described in Japanese Patent Laid-Open No. 7-78624.
The fuel cell system has a cathode-anode differential pressure gage 7 added to the fuel cell system in FIG. 2. Reaction product S3a and S3c are taken in to the cathode-anode differential pressure gage 7 and differential pressure between the cathode side gas chamber 3a and the anode side gas chamber 3c is measured to output measured result S7. A control portion 8 is connected to an output side of the cathode-anode differential pressure gage 7. The measured result S7 is input to the control portion 8 and a control signal S8 at a level proportional to the measured result S7 is output from the control portion 8. A cathode-anode differential pressure valve 9 is connected to an output side of the control portion 8. The control signal S8 is input to the cathode-anode differential pressure valve 9 and the S3c is discharged from the cathode-anode differential pressure valve 9 at an opening proportional to the control signal S8. Therefore, differential pressure between the cathode side gas chamber 3a and the anode side gas chamber 3c is kept within a permissible value and the fuel cell 3 is prevented from being damaged and destroyed. However, the fuel cell system has a problem that the fuel cell system has a cathode-anode differential pressure gage 7, the control portion 8 and the cathode-anode differential pressure valve 9 added to the fuel cell system in FIG. 2 therefore the number of parts is large and structure is complex.
To solve the above-described problem the present invention provides a fuel cell system comprising:
a supply means taking in cathode active material, supplying the cathode active material proportional to a level of a first control signal and detecting a flow rate of the cathode active material to generate a flow rate detecting signal;
a fuel cell having a cathode side gas chamber, a cathode, an anode side gas chamber, an anode and an electrolyte layer between the cathode and the anode, taking in the cathode active material supplied by the supplying means to the cathode side gas chamber, taking in given anode active material to the anode side gas chamber, discharging first and second reaction product from the cathode side gas chamber and the anode side gas chamber, respectively, and generating electromotive force between the cathode and the anode
a pressure regulating means having a pressure regulating valve regulating pressure at discharging the first reaction product on the basis of a level of a second control signal and detecting an opening of the pressure regulating valve to generate an opening detecting signal.
The input signal indicating the target electric power of the fuel cell inputs the control means and the control means decides the target value of a flow rate of the cathode active material and the target value of the opening of the pressure regulating valve in accordance with the input signal. The first control signal in accordance with the flow rate of the cathode active material and the second control signal in accordance with the target value of the opening of the pressure regulating valve are output The cathode active material is taken in to the supplying means, the flow rate of the cathode active material proportional to the level of the first control signal is supplied and the flow rate detecting signal indicating a flow rate of the cathode active material is generated. The target value of a flow rate of the cathode active material and is compared with the flow rate detecting signal by the control means, and when the flow rate detecting signal is larger than the target value of the flow rate, the first control signal for reducing the flow rate of the cathode active material is output, and when the target value of the flow rate is larger than the flow rate detecting signal, the first control signal for increasing the flow rate of the cathode active material is output.
The cathode active material supplied by the supplying means is taken in to the cathode side gas chamber of the fuel cell and the anode active material is taken in to the anode side gas chamber. The first and second reaction product are discharged from the cathode side gas chamber and the anode side gas chamber, respectively, and electromotive force is generated between the cathode and the anode. The pressure at discharging the first reaction product is adjusted by the pressure regulating valve on the basis of a level of a second control signal and an opening detecting signal indicating an opening of the pressure regulating valve is generated. The target value of the opening of the pressure regulating valve is compared with the opening detecting signal by the control means, and when the opening detecting signal is larger than the target value of the opening, the second control signal for reducing the opening is output, and when the target value of the opening is larger than the opening detecting signal, the second control signal for increasing the opening is output.
When the target electric power is reduced, e.g., in a case in which the fuel cell at operating is stopped suddenly, in the first control means, after a predetermined period of time has passed since a time of starting to reduce the level of the first control signal, the level of second control signal starts to be reduced. In the second control means, reducing speed of the level of the second control signal is decreased at a uniform ratio with respect to reducing speed of the level of the first control signal. In the third control means, after a predetermined period of time has passed since a time of starting to reduce the level of the first control signal, the level of second control signal starts to be reduced, and reducing speed of the level of the second control signal is decreased at a uniform ratio with respect to reducing speed of the level of th first control signal.